Lemma 2.1 ([1]}Theorem 2.47)
There exists a constant C such that for any real number \(s\)  and any \((p,r)\)  in \([1,\infty ]^2\) , we have, for any \((u,v)\in L^\infty \times \dot{B}_{p,r}^{s},\)
\( \Vert \dot{T}_u v\Vert _{\dot{B}_{p,r}^{s}}\le C^{1+|s|}\Vert u\Vert _{L^\infty }\Vert v\Vert _{\dot{B}_{p,r}^{s}}.\) 
